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Technology Review has a writeup on the latest advance in the lab towards an invisibility cloak made of metamaterials, described this week in Science. We've been following this technology sincethebeginning. The breakthrough is software that lets researchers design materials that are both low-loss and wideband. "The cloak that the researchers built works with wavelengths of light ranging from about 1 to 18 gigahertz — a swath as broad as the visible spectrum. No one has yet made a cloaking device that works in the visible spectrum, and those metamaterials that have been fabricated tend to work only with narrow bands of light. But a cloak that made an object invisible to light of only one color would not be of much use. Similarly, a cloaking device can't afford to be lossy: if it lets just a little bit of light reflect off the object it's supposed to cloak, it's no longer effective. The cloak that Smith built is very low loss, successfully rerouting almost all the light that hits it."

Similarly, a cloaking device can't afford to be lossy: if it lets just a little bit of light reflect off the object it's supposed to cloak, it's no longer effective.

Why would that be no longer effective? If the cloak reroutes 90% of the light, then you're left with 10% opacity, right? Sure, something that translucent would be very difficult to see, especially from a distance.

Why would that be no longer effective? If the cloak reroutes 90% of the light, then you're left with 10% opacity, right? Sure, something that translucent would be very difficult to see, especially from a distance.

The Predator still got his ass shot up good with that hand-held vulcan gun, because the soldier saw the 10% of light that he couldn't cloak.

Why would that be no longer effective? If the cloak reroutes 90% of the light, then you're left with 10% opacity, right? Sure, something that translucent would be very difficult to see, especially from a distance.

The Predator still got his ass shot up good with that hand-held vulcan gun, because the soldier saw the 10% of light that he couldn't cloak.

Yes, but if you look at it from a D&D point of view, you get a 90% miss chance, which is a game-breaking advantage.

Nah, if you have a cross-dressing fetish you should advertise it. Yeah most women will be offput, but there will be a fair number who like guys in dresses too. In fact, thats how most of the sexual crossdressers I know *got* the fetish in the first place.

Why would that be no longer effective? If the cloak reroutes 90% of the light, then you're left with 10% opacity, right? Sure, something that translucent would be very difficult to see, especially from a distance.

The Predator still got his ass shot up good with that hand-held vulcan gun, because the soldier saw the 10% of light that he couldn't cloak.

That's what you get for pissing off Jesse "the future Governor of Minnesota" Ventura.Cloaking device or not.

It is a misconception thinking you are 10% opacity. This is because the meta material bends light depending on frequency. So you will experience a hue shift under the circumstance where not all frequency are covered. In simpler term, you'll appear to be bright red or bright violet due to that 10%. That effectively turns you into a gigantic painted target that screams "shoot me!"

If visible light is being routed around the cloak, it could cause some serious navigation issues for the cloaked object. Maybe some objects (ships/aircraft) will only need a cloak that routes radar, leaving pilots to navigate by sight and dead reckoning (GPS uses radio frequencies, right?)

How is 1 to 18 gigahertz a swath as wide as the visual spectrum? It's much wider. This is around 4 octaves (ie, doublings of frequency). The visual spectrum is from 400 to 700 nanometers - not even a full octave.

Th refractive index of the material does _not_ depend on the frequency of the light wave, even though different frequencies refract at different angles (think prism). The refractive index depends upon the molecular structure of the material (and temperature to a point). Then, precise refraction can be calculated based upon the refractive index of the entering material, the exit material, and the frequency.

Given the radius of a circle you can calculate its area, but that doesn't mean they're the same thing or that you can use them interchangeably. Convertibility is not equivalence, and the article as written is wrong.

The speed of light is not constant when not in a vacuum, and the wavelength frequency conversion depends on the speed of the wave, not the speed of the wave in a vacuum. So, you can't convert the frequency in this article to wavelength without first knowing the properties of the medium the frequency was measured in.

The 'invisibility cloak' thing is right up there with 'teleportation'. Every time someone manages to 'teleport' the state of a single subatomic particle, we get a bunch of articles likening the process to Star Trek teleporters.

Do ANY of the researchers involved in these things really expect them to have invisibility or teleportation capabilities at macro scales someday? I was under the impression that neither of them had any relevance at larger scales, and while I could be wrong, it seems like the media j

"Coming soon to a sticky theater near you, Harry Potter and the Seven Sexy Spells!"

RON: "But Harry we can't go in the girl's dorm. We're gonna get in trouble."HARRY:"Shush Ron. This is important. Look there's Hermione in her underwear."RON: "What's Victoria's Secret?"HARRY:"You really are clueless aren't you?"RON: (blinks) "I never knew Hermione was so... large. Wow. Like two grapefruits!"HARRY:"I've been helping her with engorgement spe

Sigh, here we go again! Radars and optical vision do not work in remotely the same way. Creating invisibility in the two different realms is a completely different problem.

In most vision situations there are two critical factors which don't occur in the great majority of radars. The first is illumination of the target from angles other than the viewing angle (OK, there are bistatic radars, but they are not common) and the other is a background which is illuminated. Try to think about this for just a few moments. Why can't we all make ourselves invisible just by wearing matt black clothing? Well, obviously because we will stand out against the background unless we happen to be standing in front of black wall or wandering around in a coal mine. The whole point of the fictional 'invisibility cloak' is that it works in all circumstances. We can already be invisible in certain carefully controlled environments, that after all is what camouflage is all about.

But, a radar is rather like wandering about in the above mentioned coal mine, or perhaps a dark forest with a miner's lamp fixed to your head. The background is basically black and the illumination comes from the viewing direction. In this scenario, someone dress entirely in black would be effectively invisible. And that is the key point to grasp. In the world or radar we can achieve invisibility simply by making sufficiently 'black' 'paint'. The weird ability of these meta-materials to allow the illumination to pass through the target un-disturbed is of no benefit. Since we don't have a receiver on the other side of the target to detect this energy it isn't relevant. Now, sure, we can all dream up complex bistatic radars which rely on the obscuration of the signal to detect the target, but I remain to be convinced that such a thing can be made sufficiently versatile to be useful.

Can I stress that I am not suggesting the these meta-materials don't have an application in the world of radar. They seem to me to be particularly useful where one wants to remove a fixed object which obscures the view of your radar. For example, consider a radar on a ship. It may well find that in some directions its view is obscured by other parts of the superstructure. If the could cover these other bits of the ship with meta-materials such that the radar pulses could pass 'through' and back again undisturbed, then our radar's field of view would be increased. Such an application would work perfectly well with even the relatively narrow band materials presented previously.

While absorbing the radar waves works. That doesn't make being transparent to those waves useless. It's just a different method to the same problem. It has actually the advantage, as you pointed out, to be a more generic solution. "Black paint" works well today but "transparent paint" may be necessary tomorrow.

Either completely absorbing or allowing the RADAR waves to pass by undisturbed are equally valuable for RADAR cloaking. The key issue is how little of the radio waves are reflected. I'm not sure which will be more viable in the future, but sufficiently absorptive paint and structures are the winners for now.

Indeed, I couldn't agree more, and this is a property which rarely seems to be mentioned in meta-material discussions. Supposing that such a material passed 95% of the energy undisturbed and only reflected 5%. I think that this would rightly be regarded as an excellent technical achievement, and after all some glass isn't that good, but it would seem to be of limited military value. As we know from the radar equation's R^4 term, this will only reduce detection range by a little more than 50%, and you can

And with the new chinese technology, for which I can't find a link, that is a problem. They have found a way to look for momentary interruptions in consumer elecronics' signals to find a stealth aircraft. It has already been masively deployed with the excuse that it is used for censorship.

I think those that tested the hell out of relativity in the early 20th century would disagree with you. When something is so out of the ordinary or flies so much in the face of conventional wisdom, it will be tested more than something which merely "builds" on an existing concept.

You may be right, but in your purist logical thinking you are denying a fundamental attribute of humans.

A basic understanding of the spectrum (and absolutely no RTFA on my behalf) would suggest that they mean one colour of the spectrum. So if they can cloak, say, the red spectrum, you'd show up looking a different colour than your normal sort.

Imagine looking at some purple paper and then removing the red visibility/light from it. Is it still purple to your eyes?

As part of the hype, one article a while ago showed off such a photo... which was actually a normal photo of a tank, badly Photoshopped to make it look like it was fading to invisibility at one end. That kind of nonsense highlights the level of hype about this project.

As I said in reply to a similar objection, the infra-red detectors used, say, to detect body heat on a battlefield, are only sensitive to a very narrow band within that range. All you need to do is match that band.

Why should an "invisbility cloak" only be useful if it works 100%? Camouflage is being used in pretty much every war, and it's far from perfect. A cloak that "leaks" could still be great in low-light or reduced visibility settings. It doesn't have to be "perfect" to be useful. The stealth fighters aren't really radar-invisible, either. Just very difficult to detect, and for most settings that's good enough.

Visible light is roughly from 380 to 750 nm, which corresponds to frequencies of 400 to 790 THz. That's a bandwidth of 390 THz.

The article says the thingamajig in question operates over "wavelengths of light ranging from about 1 to 18 gigahertz", which I'm going to assume means frequencies from 1 to 18 GHz, which comes to a bandwidth of 17 Ghz.

390 THz / 17 GHz = 22,941.1765.

So, the visible light spectrum represents a bandwidth of only a bit more than four orders of magnitude more tha

Seeing as we're talking about relative "sizes" of bandwidths you should probably be comparing the difference between the logs of the relevent extents. The reason being that visible light extends over roughly an octave, whereas this cloak extends over roughly 4. You'd be right to say that the size of the cloak's bandwidth is four times that of the visible light spectrum, but not four orders of magnitude more. Your overall conclusion is right, the article is way off.

Yep, that's the one. Frequencies should be thought of logarithmically. You can use the musical concept of octaves in this case. 1-18 GHz is about 4.17 octaves, whereas 400-750 THz is about 0.9 octaves.

Bah, "ain't" is a perfectly valid contraction for "am not", and has been since at least 1706. (See http://www.etymonline.com/index.php?search=ain't&searchmode=none [etymonline.com]) Proscriptionists object to it largely because it's often used for "is not", or "are not", which was seen as somehow "perverting" the English language.

In fact, though, "ain't" has been used that way since at least the 19th century.

About the worst that you can say of "ain't" is that it's inappropriate for a formal register, but so are most contractions.

"But a cloak that made an object invisible to light of only one color would not be of much use." If the color corresponded closely with the wavelength of laser weapons resistant/protective eye-wear could be developed of such materials.

We have simpler and cheaper absorptive filters for that already. But, making it invisible to the IR lasers used for laser rangefinders could come in handy, although it wouldn't take long to train tank crews to lase something next to the tank rather than the tank itself.

Maybe you could make it wideband enough to defeat IR heat sensitive cameras. That would be interesting. Would it look like an absolute zero patch rather than a hot engine? Probably wouldn't take long to reprogram the missiles to home in o